This invention relates to a solar heat collector designed to transfer solar energy to a liquid. Past designs of liquid heating solar colectors have been of two general types: those pressurized throughout and those which utilize gravity to create liquid flow over an absorber surface, these being commonly known as trickle, gravity-flow or open collectors, one of the most well-known of which is the subject of U.S. Pat. No. 3,360,539 by Thomason. With regard to materials, most collectors are made of either metal or plastic.
A major advantage of trickle type collectors is that they do not require the construction of pressurized spaces or the use of extensive lengths of tubing in their manufacture, but their problems include the fact that at low liquid flow rates there is poor thermal contact between the absorber and the liquid because of non-uniform liquid distribution. To prevent evaporation and cooling, the wetted surface of the absorber must be contained in an essentially vaportight enclosure. This has been accomplished by placing one or more layers of glazing over the absorber surface. The cost of this glazing is a significant part of the collector's cost. Condensation on this glazing tends to reduce light transmission through it. Sealing the glazing economically and in a vapor-tight manner has also been a problem.
Ying-Nien Yu, in U.S. Pat. No. 3,943,911, discloses the use of a thin plastic film over the absorber surface to assist in evenly spreading the liquid over the absorber. He also proposes the use of the plastic film as the absorber itself. However, many plastics deteriorate when exposed to sunlight, and when used with a rigid frame to seal their edges, they wrinkle as the result of thermal expansion and contraction, Wrinkling disturbes the even and uniform flow of liquid. Although some plastic films are stiff and do not wrinkle unless stressed in some manner, other plastic films which might be suitable for use in a collector are very soft and must be kept under tension for them to lie flat and free of wrinkles.
A major advantage in the use of plastics in solar collectors is the absence of corrosion problems. However, low-cost plastics, such as polyethylene, deteriorate when exposed to sunlight and melt at relatively low temperatures. Consequently, the use of glazing over plastic collectors is usually hazardous because a temporary loss of liquid flow might result in the plastic being heated to the melting point. Plastics which are able to withstand relatively high temperatures and exposure to sunlight tend to be expensive. Efforts to offset the loss of strength as the result of temperature or prolonged sun exposure by increasing the thickness of the plastic in a collector also tend to reduce the efficiency of the collector, as most plastics tend to be relatively poor heat conductors. No economical way has been found to solve these problems of plastic collectors. The present invention, however, offers solutions to these problems and also provides other advantages.